Lead in Garden Soils

Key points

• All Maryland soils contain naturally-occurring background levels of lead, typically less than 50 ppm. Some garden soils, however, contain moderate to very high levels of lead that may pose a serious health risk. 
• The risk is primarily from contaminated soil brought into the home on clothing, shoes, and tools. The soil becomes mixed with house dust that is inhaled or ingested. This can result in dangerous increases in blood lead levels, particularly in infants and toddlers. Lead may also be ingested from contaminated soil clinging to harvested vegetable crops. 
• Lead has low solubility and plant uptake in soil with adequate phosphorous and organic matter and a 6.0-7.0 pH. The risk from plant uptake of lead is low; the main concern is soil ingestion.
• All vegetable garden soils should be tested for lead.

Sources of lead

• Chipping or peeling paint around older structures will raise the lead level in the soils directly adjacent to the building.
• Even today, when an old building is demolished, the soil can become contaminated with lead from old lead paint. In the 1950s, cheaper titanium pigments largely replaced lead pigments. Federal restrictions were not imposed until the late 1970s.
• Soil can be contaminated with lead from several other sources - industrial sites, leaded fuels, old lead plumbing pipes, or even old orchard sites in production when lead arsenate was used as a pesticide.
• Lead accumulates in the upper 8 inches of the soil and is highly immobile.
• Contamination is long-term. High soil lead levels do not decline over time but lead can be made less available through some best gardening practices (see below).

Lead health risks

• Lead is not required in our diet or environment.
• At very low levels that naturally occur in soils (10-50 ppm), no detrimental health effects have been noted. But higher soil lead levels can raise the body's lead level without producing any obvious physical symptoms.
• Young children under the age of 6 and pregnant women are at the greatest risk.
• As a group, children exposed to higher levels of lead have lower IQs and may experience permanent learning disabilities and behavioral disorders when compared to children not exposed to lead.

Lead and soil testing

• Elevated lead levels are more common in urban neighborhoods, but suburban and rural soils may also be contaminated.
• The greatest lead concentration is in the top 1 to 2 inches of soil. Children's play areas or vegetable gardens should be sampled separately. Avoid mixing several sites into one sample.
• Testing for lead will help to evaluate the potential risk to health. The risk is based on exposure.
• Soil samples should be taken from several areas to determine the location of the contamination.
• All vegetable garden soils should be tested for lead. Soil laboratory results will be returned listing the parts per million (ppm) of lead from either an extracted or total lead test, or both. Pay careful attention to the total lead values.
• No legal regulations for soil lead levels are in effect. However, the most current U.S. Environmental Protection Agency (EPA) recommendation is to avoid growing vegetables in soil with a total level above 400 ppm (see below).

Soil testing labs that test for lead

The following laboratories will test soil for lead. This list is by no means complete and is intended for reference only. Mention of businesses in this publication does not constitute an endorsement by University of Maryland Extension. Check the websites or contact the individual labs for their procedures and charges before sending the sample.

• University of Delaware Soil Testing Program
  152 Townsend Hall
  531 S. College Ave.
  Newark, DE 19716
  Phone: 302-831-1392
  Lead included in basic "Home Lawn and Garden Soil Test”
   
• University of Massachusetts Amherst
  UMass Soil & Plant Nutrient Testing Lab
  203 Paige Laboratory
  161 Holdsworth WayAmherst, MA 01003
  Phone: 413-545-2311
  Lead included in basic soil test
   
• Pennsylvania Agricultural Analytic Services
  111 Ag Analytical Services Lab
  University Park, PA 16802
  Phone: 814-863-0841
  Look under 'environmental soil testing'

Reducing the health risks of lead contaminated soil

• Don't locate food gardens next to a busy road or a home built prior to 1940 with a painted wooden exterior.
• In heavily contaminated soils adjacent to a residence, plant trees, shrubs or perennials, and mulch the area to keep the soil covered. Soil removal (by a certified contractor) and replacement should be considered if the total soil lead level is over 5,000 ppm.
• Don't allow young children to play in contaminated soils. Frequent hand washing and rinsing outside toys will reduce the amount of soil ingested. Always wash hands before eating meals or snacks. Have family members leave outdoor shoes in a cardboard box at the door, to avoid spreading lead-contaminated dust through the home. Rinse and launder gardening clothing promptly. Mulch play areas with wood chips or other soft materials to reduce soil dust.
• Parents of children under age 6 living in areas with contaminated soils should consult their physician. A blood test to monitor lead levels may be recommended.

Gardening in soil with total lead levels 50 ppm to 400 ppm

• All vegetable and fruit crops can be safely grown.
• The amount of available lead in soil is affected by the soil pH, organic matter, phosphorus content of the soil, and total soil lead level. To reduce availability and uptake by plants, adjust the pH of the soil to a 6.0 to 7.0 range. Increase soil organic matter by adding compost, manure, leaf mold, and grass clippings (un-treated) to the gardening site. The top 8 inches of soil should be at least one third organic matter by volume. Add phosphorus to the soil as recommended by a soil test.
• Fruits (tomato, pepper, cucumber) are much less likely to contain lead compared to leafy vegetables and root vegetables. Carrots easily take up lead and store it in the edible roots.
• Contaminated soil particles can cling to or become embedded in leafy greens and root crops.
• Wash all vegetables and peel all root crops before they are cooked and eaten. Remove the outer wrapper leaves of cabbage.
• If possible, wash off excess soil from root and leaf crops outside the house to prevent bringing contaminated soil into your home.

Gardening in soil with total lead levels 401 ppm to 1,000 ppm

• Fruiting crops (tomato, pepper, squash, sweetcorn, etc.) can be safely grown. Follow guidelines above.
• Leafy greens and root crops should be grown in containers or in raised beds placed on top of contaminated soil and filled with a mixture of clean topsoil (low in lead) and compost, to at least an 8-inch depth.